This invention relates to a glass comprised primarily of silica. More particularly, this invention relates to a silica glass doped with constituents to provide high viscosity and resistance to diffusion of hydrogen and alkali ions therethrough. The glass of the invention is particularly well-suited to use in lamp and semiconductor applications. Throughout the specification, numerous references will be made to use of the glass in construction of lamp envelopes or in crucibles and tubes for semiconductor applications. However, it should be realized that the inventive glass is also suitable to most glass applications including window glass for high temperature and space environments.
This invention relates to a glass formed of natural or synthetic silica having very low levels of impurities yet including dopants that effectively improve viscosity characteristics and trap monovalent ions. This invention is particularly suited to the use of synthetic silica because of its low impurity levels.
Silica glass has been used in metal-halide, halogen and mercury lamps because of its excellent mechanical strengths and its ability to handle high operating temperatures. In addition, fused silica is becoming an important constituent of the ever-expanding semiconductor and fiber optics industries wherein high purity and resistance to high temperatures are requisite characteristics.
A particular requirement in the semiconductor industry is a desire for low levels of impurities. Accordingly, efforts are frequently made to reduce impurities to levels as low as possible. However, a difficulty often encountered in utilizing the low impurity synthetic silica is a tendency towards a low viscosity. Although a low viscosity may benefit fabricators who rework silica products, many fabricators require a fused silica having a higher viscosity. The semiconductor industry is an example of an industry which requires a high viscosity yet low impurity levels. The present invention has identified a plurality of dopants which improve viscosity characteristics without adding unacceptable contaminants to the glass composition.
In addition to the semiconductor industry, low impurity level, high viscosity silica will have great suitability in the manufacture of lamp envelopes. Particularly, metal halide arc discharge lamps in which the glass composition of this invention is beneficial when utilized to form the arc chamber, include, but are not limited to U.S. Pat. Nos. 4,047,067 and 4,918,352 (electrode), and 5,032,762 (electrodeless), the disclosures of which are herein incorporated by reference. Metal halide lamps of this type are generally comprised of an arc discharge chamber surrounded by a protective envelope. The arc chamber includes a fill of light emitting metals including sodium and rare earth elements such as scandium, indium, dysprosium, neodymium, praseodymium, cerium, and thorium in the form of a halide, optionally mercury, and optionally an inert gas such as krypton or argon. U.S. Pat. No. 4,798,895, herein incorporated by reference, describes a representative metal halide dose which when used in combination with an envelope comprised of sodium resistant glass of the present invention, creates a superior lamp.
It has been found that the life of metal halide lamps is frequently limited by the loss of the monovalent sodium ions, particularly from the metal halide fill during lamp operation via ion diffusion through the arc chamber. More particularly, fused quartz and synthetic silica are relatively porous to ion diffusion, and during lamp operation, energetic monovalent ions pass from the arc plasma through the chamber wall and condense in the region between the arc chamber and the outer jacket or envelope of the lamp. In the case of sodium, the lost sodium is then unavailable to the arc discharge and can no longer contribute its characteristic emissions, causing the light output to gradually diminish, and causing the color to shift from white towards blue. In addition, the arc becomes more constricted, and in a horizontally operated lamp, the arc may bow against and soften the arc chamber wall. Sodium loss may also cause the operating voltage of the lamp to increase to the point where the arc can no longer be sustained by the ballast and failure of the lamp may result.
In an attempt to reduce the effects of sodium diffusion through the arc chamber, the skilled artisan has historically relied on coating the arc chamber with sodium diffusion resistant materials. Attempts to solve diffusion problems have included depositing aluminum silicate and titanium silicate layers on the outside surfaces of the arc tube, as described in U.S. Pat. Nos. 4,047,067 and 4,017,163 respectively. Alternatively, U.S. Reissue Pat. No. 30,165 discloses applying a vitreous metal phosphate and arsenate coating on the inner surface of the arc tube. In contrast, U.S. Pat. No. 5,032,762 discloses beryllium oxide coatings.
While these methods have met with success in reducing sodium diffusion, the methods also require additional processing steps associated with applying a coating. Furthermore, the lamp's high temperature of operation, and frequently corrosive environment, may destroy the adherence between coating and arc chamber. Moreover, cracking and/or peeling can result, exposing the quartz to sodium ions and allowing sodium diffusion to occur. Accordingly, it would be desirable in the art to have a glass material which reduces sodium diffusion without the application of additional coatings.